I. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly to methods and apparatus for providing improved bandwidth utilization of a return link in satellite communication systems by access terminals using an orthogonal code division multiple access approach in the return link.
II. Background
Various satellite communication systems have been developed over the years. One early system architecture is referred to as Time Division Multiple Access (TDMA) and is characterized by assignment of time slots in a communication channel to each of a plurality of terminals, and with communication with the terminals taking place in the specifically assigned time slots. An improved system architecture is referred to as Code Division Multiple Access (CDMA). CDMA based communication systems generally provide greater bandwidth efficiency than do TDMA based communication systems. Many CDMA based satellite communication systems typically operate in a CDMA mode in the forward link (FL) direction and in a TDMA mode in the return link (RL) direction.
There are two main modes of CDMA, asynchronous CDMA and synchronous orthogonal CDMA. In the asynchronous mode of CDMA operation, the signals from different terminals are not synchronized and, therefore, cause interference to each other. In the synchronous mode of operation, the transmission signals from different terminals are timed such as to arrive synchronously at the receiver. In the synchronous mode of CDMA, if orthogonal codes are used to distinguish different terminal transmissions, then there is no cross-interference among the different signals received from terminals. In this case, one achieves higher bandwidth efficiency due to reduced interference. On the forward link, orthogonality among different codes is maintained because all signals originate at the same location, namely at the gateway. On the return link, however, without a synchronization mechanism, signals from different terminals arrive asynchronously at the gateway due to the their different propagation delays. What is needed are methods and apparatus for operating an orthogonal CDMA based return link for satellite communication systems.